1. Field of the Invention
The present invention relates generally to an image processing apparatus and, more particularly, to an image processing apparatus for generating output pixel data by halftoning input pixel data.
2. Description of the Related Art
A dither method is known as one conventional means utilized for reproducing half tone in the image forming sections of digital printers, digital facsimiles or the like of known types.
In a normal dither method, a dither matrix of m.times.n is prepared, and a binarized block of m.times.n is formed by comparing a group of input pixel data with corresponding thresholds in the matrix. In this manner, a half-tone image is reproduced in a pseudoform.
However, the number of steps of tone representation is restricted by the matrix size of the dither matrix. For example, in the case of tone representation in sixteen steps (a 4.times.4 dither matrix or the like), there is a problem in that a pseudo-outline may occur in an output image and a good output image cannot be obtained.
In such a situation, a method which has recently received special attention is an error diffusion method.
This method was proposed by Floyd and Steinberg in the paper entitled "An Adaptive Algorithm for Spatial Gray Scale," SID DIGEST 1975. The error diffusion method is superior to the dither method in resolution and tone representation.
In the error diffusion method, since an error which occurs when a binary output pixel is determined is sequentially spatially calculated in a cumulative manner, it is possible to make the density of an input image correspond to the density of an output image. Unlike the dither process, the number of steps of tone representation is not limited by the size of the dither matrix, and it is therefore possible to improve tone reproduction and resolution, which cannot both always be compatibly satisfied using the dither process.
However, such an error diffusion method has a problem of causing the phenomenon in which, if a low-density portion is present at the portion of an image which corresponds to the beginning of the process, no dot is plotted and a white void is therefore formed in the low-density portion. This phenomenon likewise occurs in a low-density portion near an edge portion.
For example, as shown in FIG. 29, if the density of an area a of an original document is continuously low, white pixels will be continuously represented in a binarized output. All the errors that occur as the result of binarization become plus, and the errors are distributed to and accumulated in unprocessed neighboring pixels. As a result, for example in an area b, the accumulated errors exceed the thresholds of respective objective pixels, and black dots such as those shown in FIG. 29 are formed close to one another, thereby causing a deterioration in image quality.
To cope with the above problem, a method of varying a threshold on the basis of which an error is diffused is considered. In this method, however, dots which are present in an edge portion of an image, particularly in the peripheral portion of a character, a line drawing or the like are notched and the image quality of a character or a line drawing is thus degraded to a remarkable extent. In addition, in a uniform low-density portion, no dot is plotted and a roughly granular texture which may offend the eye may appear.
In addition to the methods described above, the assignee of the present invention has previously disclosed the following methods having similar purports.
In U.S. patent application Ser. No. 07/270,809 filed on Nov. 14, 1988, a variable-threshold process and a window process are disclosed. In the variable-threshold process, a threshold for use in binarization is varied in accordance with the density of an object pixel, or objective pixel (throughout the specification and claims, these two terms are used interchangeably) and an error diffusion method is carried out on the basis of the varied threshold. In the window process, a binarized area which has been processed is referred to and, if any dot is present in the area, no dot is plotted.
In U.S. Pat. No. 4,958,238, an edge detecting process is disclosed in which an error diffusion method is carried out in such a manner that no negative error is diffused into an area occupied by pixels which is located in the edge portion of an image.
In U.S. Pat. No. 4,969,252, a remainder correcting process is disclosed in which a remainder occurring when an error is diffused into an area occupied by pixels which neighbors on an objective pixel.